You need access to the back of the switch. It will likely be a 2 or 3 pole switch, depending. You'll need a multimeter that can test either continuity, or OHMS.
If it's a 2 pole, easy, throw a lead on each contact, and see if there's infinite, or 0 resistance. Flick the switch a few times to see if that state changes.
If it's a 3 pole, one pole will be ground (usually the middle one) and the other two will be varying power, which means you'll see <infinite ohms resistance, but it won't be wide open. Alternatively, it could be a single power, with 2 ground. Regardless, it should be easy to test with a multimeter, however you'll need access to the back of the switch, whether that means removing it, or popping the entire piece of trim out.
Do they look like this?